what is the difference between using the same value for two case labels and using several case labels for a single case?

I was reading from Bjarne's Programming and Principles using C++.

I ran into the following details about switch:

You can use several case labels for a single case.

You cannot use the same value for two case labels.

I think I clearly understand the 2. one. It should mean that:

switch (a) {

  case 'c':{//some code} 

  case 'c':{//some (different) code} 
} 

Is not legal.

However, I am not sure if the first one means, that if case no.1 and case no.2 and so on are different then I can have any number of cases(, of course them being constant expressions) or it means that I can have any number of cases, but some of them doing the same thing.

I have found a similar question regarding this: multiple label value in C switch case

Do I interpret it right? If not, what is it I am missing or get wrong?

This is allowed:

switch ( expression )
{
    case 1:
    case 2: 
    case 3:
          //some code
          break;
    //...
}

And this is NOT allowed:

switch ( expression )
{
    case 1:
          //some code
          break;
    case 1:
          //some other code
          break;
    //...
}   

So, it's not allowed to use the same value for two or more case labels.